A source of coherent tuneable blue, green, or yellow light would have a substantial number of potential applications. A means of converting readily available red laser diode radiation into blue, green, or yellow light would also be advantageous. However, there are presently no solid state tuneable lasers operating at these wavelengths. Tuneable optical parametric oscillators operating at very high peak powers are too expensive to use in everyday optical fibre systems.
Previous fibre optic parametric oscillators have used a pump the in the anomalous dispersion region close to the zero group velocity dispersion (GVD) of the fibre, where the parametric amplification gain is also called the modulation instability. The two wavelengths generated by this method are closely spaced around the pump wavelength, and cannot be used to extend the tuning range far from the pump wavelength.
Conventional optical fibres have a central core surrounded by glass with a slightly different composition to the central core. Typically the central core is doped so that it has a different refractive index to the surrounding glass. Light travelling down the central core of the fibre is confined by the interface between the fibre and the surrounding glass.
In the last few years a non-standard type of optical fibre has been demonstrated, called the photonic crystal fibre. Typically, this is made from a single solid, and substantially transparent, material such as silica within which is embedded an array of air holes, running parallel to the fibre axis and extending the full length of the fibre. The arrangement of air holes in the array may be periodic but need not be and the air holes may be filled with a material other than air. A defect, for example, in the form of a single missing air hole within the regular array forms a region of raised refractive index within which light is guided, in a manner analogous to total-internal-reflection guiding in standard fibres. Another mechanism for guiding light in a photonic crystal fibre is based on photonic-band-gap effects rather than total internal reflection. Photonic-band-gap guidance can be obtained by suitable design of the array of air holes. Light with particular propagation constants can be confined to an air core and will propagate therein. A photonic crystal fibre can be fabricated by stacking glass canes, some of which are capillaries on a macroscopic scale, into the required shape, and then holding them in place while fusing them together and drawing them down into a fibre. A photonic crystal fibre has unusual properties such as the ability to guide light in a single-mode over a very broad range of wavelengths, and to guide light having a relatively large mode area which remains single-mode.
Photonic crystal fibres have the potential to greatly increase the number of fibre based optical devices as they have a wide range of properties that can be engineered to suit the application.